نقش منطقه همگرایی بین حاره‏ای (itcz) در تکوین و الگوی گسترش زبانه کم‏فشار سودانی در بارش‏های فراگیر و شدید جنوب ایران

بارش‏های همرفتی شدید در نتیجه پتانسیل رطوبت‏پذیری بالا و دسترسی به منابع رطوبتی دریاهای گرم‏جنوبی از ویژگی‏های بارشی سامانه کم‏فشار سودانی است که به خصوص بر روی کشورهای حاشیه جنوبی خلیج‏فارس، نیمه‏جنوبی و جنوب‏غربی ایران بسیار مشهود است. برخی از محققین منشا اولیه این سامانه را گسترش شمال‏سوی منطقه همگرایی بین حاره‏ای (itcz) در محدوده °25 تا °35 شرقی (آفریقای شرقی) و ایجاد یک هسته حرارتی بر روی جنوب سودان و اتیوپی می‏دانند. لذا هدف این تحقیق شناسایی ارتباط و انطباق گسترش شمال‏سوی زبانه کم‏فشار سودانی با الگوی گسترش شمال‏سوی itcz در هنگام وقوع بارش‏های فراگیر و شدید نیمه‏جنوبی ایران است. براساس معیارهای انتخابی، 86 سامانه‏ بارشی فراگیر در منطقه شناسایی شد. پس از ترسیم موقعیت متوسط ماهانه itcz، الگوی هسته مرکزی، اولین منحنی بسته شده و گسترش زبانه کم‏فشارسودانی برای روزهای بارشی سامانه‏ها، نتایج زیر حاصل شد. در تمام ماه‏ها زبانه‏ای از  itczدر محدوده شاخ آفریقا با راستای جنوب‏غربیشمال‏شرقی تا شمال یمن گسترش یافته است. الگوی گسترش زبانه‏های کم‏فشار سودان، پراکنش هسته‏ها و پهنه گسترش اولین منحنی بسته، کاملا با الگوی ماهانه گسترش itcz انطباق دارد. این مسئله نقش غیرقابل ‏انکارitcz  را در تکوین، گسترش و تامین رطوبت سامانه‏های بارشی نشان می‏دهد. در ماه ژانویه تعداد سامانه‏های بارشی، پهنه اثر پراکنش و عمق نفوذ محور سامانه‏های ورودی به نیمه‏جنوبی ایران افزایش اما در ماه‏های نوامبر و آوریل کاهش یافته است. این مساله می‏تواند به ترمودینامیک ضعیف‏تر سامانه‏سودانی در ابتدای شروع فعالیت،گسترش ضعیف itcz بر روی منطقه بدلیل استقرار غرب‏سوی واچرخند عربستان در ابتدای فصل بارشی و برگشت غرب‏سوی آن در انتهای دوره بارشی مرتبط باشد.

The role of InterTropical Convergence Zone (ITCZ) in the development and pattern of Sudanese low pressure trough expansion in pervasive and severe rainfall in Southern Iran

Extended abstractIntroductionSudanese lowpressure is a system with tropical characteristics. Its precipitation characteristics are very similar to that of tropical systems. This is especially evident in the countries on the southern shores of the Persian Gulf and in the southern and southwestern parts of Iran. Heavy rainfall of this system results in high moisture potential and access to the resources of the warm southern seas. Some researchers believe that the system originated in the northward expansion of the InterTropical Convergence Zone (ITCZ) in the range of 25 to 35° east (in East Africa) and created a thermal nucleus in southern Sudan and Ethiopia (Lashkari, 1996; 2001). The aim of this study is to identify the adaptation of northward expansion of the Sudanese lowpressure trough with the ITCZ northward expansion pattern during pervasive and severe rainfall in the cold season in the southern half of Iran.Materials and methodsTwo categories of data were used for this study. These data included daily precipitation data from the Iranian Meteorological Organization and the ERA interim gridded data which included sea level pressure and the specific humidity of the 700 HP of the ECMWF. Second category data with horizontal resolution of 0.5 × 0.5° during 19972017 statistical period were prepared. Subsequently, based on the selected criteria, 86 pervasive and severe rainfall systems were identified. Then, the ITCZ’s average monthly position, the central core pattern, the area of the first closed curve, and the expansion of the Sudanese lowpressure trough for all 86 days of rainfall were plotted. In the final step, the position of the ITCZ was identified using a special humidity variable of 700 HP in the tropical region for the systems of each month.Results and discussionThis study has been examining the degree of adaptation and the role of the ITCZ in the formation, expansion and moisture supply of rainfall by Sudanese systems in three general forms for selected precipitation samples.Expansion pattern of the Sudanese lowpressure coreDuring all months, the region between the eastern border of Sudan and Ethiopia is the focus of Sudan’s lowpressure core. In November, December, March and April the pattern of distribution of lowpressure cores show very good adaptation with the expansion pattern of the ITCZ. However in January and February, the cores become condensed in the southeastern part of Sudan.Expansion pattern Sudanese lowpressure first closed curveThe pattern of expansion of Sudanese lowpressure first closed curve shows very good adaptation with the trough pattern of the ITCZ in all months. The adaptation of the ITCZ trough and the pattern of expansion of the lowpressure internal core indicate that it plays a significant role in the strengthening and flowing pattern of Sudanese lowpressure.Synoptic expansion pattern of Sudanese lowpressure trough axisIn all cases and all months, the extension of the lowpressure trough, especially over the Red Sea and the Bab alMandeb Strait, and even part of the Arabian Peninsula is in adaptation with the extension of the ITCZ trough.ConclusionDuring all the months of the rainy season, an ITCZ trough with a southwestnortheast direction extends from the Horn of Africa to northern Yemen. The pattern of expansion of Sudanese lowpressure trough, the pattern of nucleus distribution and the extent of expansion of the first closed curve are fully in adaptation with the monthly pattern of ITCZ expansion. This shows the undeniable role of ITCZ in the formation, expansion and moisture supply of rainfall systems. In January, in all three cases, the number of precipitation systems, the distribution effect area and the penetration depth of the input systems to the southern half of Iran are of primary importance. This could be related to the weaker thermodynamics of the Sudanese system at the beginning of its activity, the weak expansion of ITCZ in the region due to the westward position of the Arabian anticyclone at the beginning of the rainy season and the westward return at the end of the rainy season.